II. Earth and Sun

1. Earth rotates on its axis once every 24 hours
2. receives light when facing the sun (day)
3. circle of illumination
   

1. Observations:
1. opposite seasons in northern and southern hemispheres
2. cold in winter and warm in summer
   
3. more hours of daylight in summer than winter
4. variation is greaest near poles and barely noticeable near equator
2. Explanation:  During part of the year (April - August) the northern hemisphere faces the sun more directly than the S. hemisphere so it receives more sunlight and has hotter temperatures and more hours of daylight (summer).  During other parts of the year (October - February) the S. hemisphere faces the sun more directly so it has summer while the N. hemisphere is having winter.
   
3. Causes: (fig 1-23)
   
1. Reasons why one hemisphere faces the sun more directly at any given time:
• Rotation axis of the Earth is tilted 23 ½ ° away from perpendicular to plane of ecliptic (fig 1-22)
  
• Earth rotates (orbits) around the sun.
  
• As the Earth revolves around the sun, the N pole constantly points to the N. star. (rotational parallelism or rotational polarity)
2. Reasons why the hemisphere that faces the sun more directly is warmer:
• angle of insolation (fig 4-19)
• amount of atmospheric filtering (fig 4-21)
• hours of daylight
  
4. Special dates: (fig 1-24 to 1-27) (Table 1-6)
   
1. solstices
• sun shines directly on 23 ½° latitude (Tropic of Cancer or Tropic of Capricorn)
• longest and shortest days of the year (Dec. 21, June 21)
• total light or dark above 66 ½° (Arctic circle or Antarctic circle)
direct line between Earth/Sun/North Star
2. equinoxes
   • sun shines directly on equator
   • equal day and night (12 hours each)(March 20 and Sept 22)
     
   • equal everywhere on the planet
   • 90° angle between Earth, Sun and N. Star